Ceramic glaze composition



United, States Patent 2,972,543 CERAMIC GLAZE COL [POSITION Malcolm D. Beals, Fanwood, and Karsten H. Styl1r, In, Metuchen, N.J., assignors to National Lead Company, New York, N.Y., a corporation of New Jersey No Drawing. Filed Aug. 8, 1957, Ser. No. 676,942

4 Claims. (Cl. 106-48) This invention relates to a ceramic glaze. More specifically, this invention relates to a white-opacified ceramic glaze, opacified and whitened with TiO In particular, this invention relates especially to a self opacified titania-containing glaze of the alkali boroalumino-silicate type. This application is a continuation-inpart of our application Serial No. 501,451, filed April 14, 1955, which is entitled Ceramic Glaze Composition. Serial No. 501,451, now abandoned, is a continuation-inpart of our appliaction Serial No. 381,959, filed September 23, 1953, now abandoned.

To be useful in the coating of ceramic bodies a glaze must exhibit certain properties. It is necessary, first of all, that the glaze have a coeflicient of thermal expansion similar to that of the substrate ceramic material; otherwise undue stresses will be set up during the cooling of the fired piece resulting in spalling, cracking, chipping or crazing. Aside from the necessary limitations on the coeflicient of expansion other properties are desirable in a decorative 'glaze. Among these may be mentioned high opacity, gloss, durability at a low maturing temperature, and purity of color. Particularly in the case of a white glaze, a pure white color is to be desired. Such a base color lends itself readily to the production, if desired, of pastel shades by the addition of minor amounts of "tinting agents to the formulation.

Among the opacifying agents heretofore used for the production of white glazes are ziraconia, ZrO and tin oxide, S110 These agents, while generally satisfactory, sulfer from certain serious disadvantages, among which may be mentioned their high cost and relatively low opacifying power. Thus, when using ZrO as the opacitying agent, it is customary to employ a glaze composition containing from 10 to 15 mole percent ZrO and apply the same to the substrate material atan'application weight of 60-70 grams per square foot. Similarly in the case'of tin oxide glazes, it is usualto employ from l0'to 15 mole percent SnO and apply the glaze at an application weight of 50-80 grams per square foot. With the titania-opacified glaze composition hereinafter described, on the other hand, a glaze containing even somewhat lower than 8 mole percent TiO may be applied at a weight of 20 to 40 grams per square foot, and still give complete opacification.

The use of Zr0 and SnO- moreover, sulfer from another disadvantage in that they are not adapted for use in the self-opacifying or recrystallizing type of glaze.

The disadvantages inherent in the use of ZrO and SnO opacifiers were overcome to some extent by the enamels disclosed in German Patent No. 688,787 (March 1, 1940), wherein was taught a high-alumina frit composition of the recrystallizing or self-opacified type, using .provide a white ceramic glaze of high opacity.

titania as the opacifier. The enamels therein disclosed, however, also suffered from certain disadvantages, most notably the undesirably high solubility of TiO;,, in the vitreous matrix (which prompted the inventor to recommend incorporation of additional TiO as a mill addition) and excessive fluidity of the glaze composition at ordinary maturing temperatures. These glazes were also subject to an undesirable tendency to craze on cooling of the fired article.

It is therefore an object of the present invention to provide an improved ceramic glaze. It is a further object to provide a ceramic glaze having a low coefiicient of expansion, suitable for application to low-coefficient of expansion ceramic materials. A still further object is to provide a glaze having improved viscosity characteristics at maturing temperatures. An additional object is to Still another object is to provide a durable, white, opaque ceramic glaze having a relatively low maturing temperature. Other objects and advantages will become apparent from the following more complete description and claims.

In its broader aspects this invention contemplates a ceramic glaze comprising SiO B 0 TiO A1 0 and fluxing oxides, said A1 0 being present in amounts of at least 5 mole percent, said fluxing oxides being present in amount from 10 to 18 mole percent and selected from the group consisting of Na o in amount no greater than 9 mole percent together with at least one of the oxides K 0, CaO, MgO, SrO, BaO, PbO, ZnO, Li O, CdO and ZrO In a particularly desirable embodiment, this invention contemplates a glaze of the type above described wherein said fluxing oxides comprise Na O in amount from 4 to 9 mole percent and at least one oxide selected from the group consisting of K 0 in amount up to 8 mole percent, CaO in amount up to 4 mole percent, SrO in amount up to 4 mole percent, MgO in amount up to 8 mole percent, B20 in amount up to 6 mole percent, PhD in amount up to 6 mole percent, ZnO in amount up to 10 mole percent, Li O in amount up to 8 mole percent, Cd() in amount up to 4 mole percent and ZrO in amount up to 4 mole percent.

In order-for the Ti0 to recrystallize properly during the final firing operation and give the desired whiteness and opacification, it is important that the A1 0 content be atleast 5 mole percent. Higher amounts of Al O up:to about 14 mole percent, may be employed if desired.

relatively wide limits, as will be understood to those skilled in the ceramic arts. In general, however, the SiO will be in the range from 50 to 65 mole percent, B 0 will be between 6 and 12 mole percent, and TiO will be between 5 and 25 mole percent. It is preferred, however, to have the TiO content between 5 and 16 mole percent, since amounts in this range give optimum TiO particle size. Typical formulations with respect to the relative proportions of the oxides other than the fluxing oxide may be found for example in German Patent No. 688,787. 7 Another particularly desirable embodiment of the instant invention contemplates a glaze of the above type which further comprises from 1 to 2 mole percent of P 0 Another embodiment is a glaze which further comprises from 1 to 5 mole percentF Throughout the above discussion, the percentages discussed are mole percentages based on the sum of the S1103, B303, A1303, and oxides.

A ceramic glaze having a composition as just described may be applied to common ceramic material such as whiteware bodies, terra cotta, earthenware and structural clay bodies and matured at temperatures in the neighborhood of 850-1050 C. to produce a highly opaque white glaze. The glaze is of the recrystallizing type by which is meant that it is pigmented by repre cipitation during the maturing heat of titania originally present as a melted-in constituent of the frit. It may be applied by preparing a frit having a composition within the ranges above specified, fusing the batch to provide complete intersolution, quenching in water and milling. The milled frit, usually with the addition of small amounts of clay and electrolytes, is applied to the ceramic substrate in any ordinary manner such as by dipping, brushing or spraying.

For optimum results, the proportions of the various constituents should be kept within the ranges above specified, to avoid certain undesirable results. For example, if the A1 content is too low, the viscosity of the glaze in the maturing range is too low thus resulting in coarser TiO particles in the glaze which reduce the opacity of the system. Moreover, if the total alkali content is appreciably higher than that specified, the glaze is excessively fluid and the recrystallized or precipitated particles of T iO become larger than pigmentary size and tend to develop yellow color tones as well as decreasing the opacity of the glaze. If the total alkali is too low, on the other hand, the smelting temperature of the frit becomes excessively high and it becomes difiicult or irnpractical to secure desirable frits at economical fritting temperatures. When the Na O, which is a very active flux, is too high, excessively high solubility of TiO in the glaze results, as well as excessive fluidity in the glassy matrix. This results again in inadequate opacification, excessive particle size growth and consequent yellowing. Also, the use of high-soda fluxes tends to produce a glaze with a high coefiicient of thermal expansion, which tends to cause crazing upon cooling of the fired article. Replacement of a portion of the soda with other fluxing oxides according to the present invention tends to reduce the coefficient of expansion and consequently the tendency of the glaze to craze on cooling. Such other fluxes are less violent and, therefore, do not affect the coefficient of-expansion as much as an excess of Na O does. Frits containing less than about 4.0 mole percent Na O, on the other hand, can be smelted only with difliculty and glazes which show poor flow characteristics on the ceramic substrate result so that poor surfaces are obtained on the fired specimens. Additions of K 0, MgO, CaO and Li O in amounts higher than those above specified tend to produce deleterious effects on the gloss of the fired glaze, while excessively large additions of BaO, PbO, Li O or CdO all produce yellow colorations due to excessive particle growth of the reprecipitated TiO Aside from the color and gloss effects, excessive amounts of alkali are further disadvantageous in that they reduce the acid resistance of the fired glaze, the acid resistance is normally excellent when the composition is kept within the specified ranges. In order to further illustrate the nature and use of glazes according to the present invention, the following examples are presented:

Example I A frit composition was prepared according to the preferred method by melting together for about 1% hours at about 1550 C. a mixture of the following ingredients:

Parts by weight The calculated composition of the resulting frit, ex-

pressedin both mole and weight percentages, was as follows:

Component Mole Weight Percent Percent sio, 04.2 51.0 B os 8.6 8.9 A1201 8.6 12.9 T10. 8.0 10.1 1011 0.---

These components expressed in the' Seger formula are as follows:

0.600 Na O 0.860 B203 6.420 sto 0.400 K20 0.860 A1203 0.860 "rio When complete intersolution was achieved, and the frit composition was clear, the frit was quenched in water.

One hundred parts of this quenched frit were ballmilled for 18 hours along with 4 parts of clay, 0.25 part NaNO 0.25 part of K CO and 40 parts of demineralized water. The milled frit was then strained through a 200 mesh screen to insure the absence of any coarse particles. The strained slip was well-dispersed and of sprayable consistency.

The milled slip was then sprayed onto a bisqued wall tile panel at a dry application weight of about 30 grams per square foot, dried for 4 hours at C. and then fired under oxidizing conditions (air atmosphere) at 1000 C. The kiln required about six hours to reach this temperature, and as soon as the maximum temperature was reached, heating was discontinued and the kiln was allowed to cool for 14 hours. The resulting glaze was smooth, glossy, continuous and of excellent white color.

Example 2 A frit was prepared according to substantially the same procedure as described in Example 1, using the following batch composition;

Parts by weight Potters flint 160.0 Alumina 36.3 TiO 28.5 Borax 67.8 Soda ash 8.9 Magnesium carbonate 30.7

Calculated oxide composition of the resulting frit was as follows:

These components expressed in the Seger formula are as follows:

0.429 Na O 0.586 B203 4.386 sio 0.571 MgO 0.586 A1203 0.586 'rio When intersolution was achieved and a clear solution obtained, the frit was quenched in water. This frit, after milling to minus 200 mesh was formed into a slip as described in Example 1 and was applied by spraying *onto abisqued "tile slab at 30 pounds per square foot and after drying for 4 hours the slab was fired according to the procedure of Example 1 to a temperature of 950 Cpand cooled to room-temperature in '14hours. The results obtained were similar to-those described Example 1.

" Example 3 r Another frit was prepared according to the same procedure described in Example 1 from the following-ingredients: ,r. n brw s These components expressed in the Seger formula are as follows:

0.286 Na O 0.357 MgO 0.357 BaO 0.586 12. 0 4.386 $10.2 0.586 A1203 0.586 Tio When intersolution was complete the frit was quenched in water. The frit was then milled, made into .a slip, sprayed onto a tile panel and dried according to the procedure described in Example 1. The dried tile panel was then fired to a temperature of 975 C. according to the procedure described in Example 1 and was cooled to room temperatureover a period of 14 hours. The resulting glaze was smooth. continuous, glossy and possessed an excellent white color.

Example 4 A frit was prepared in the manner described in Example 1 using the following ingredients:'

, Parts by weight Potters flint 16 .0 Alumina 38.1 Ti0 30.0 Borax 71.2 Soda ash .8.5 Potassium carbonate L 12.0 Calcite 17.3

Calculated oxide composition of the frit was as follows:

Mole Weight;

percent percent 810- 62. 8 56. 5 13 0 8. 4 8.8 A1203 8. 4 12. 8 T1Oz 8. 4 10. 1 NaaO- 6. 0 5.6 K10 2.0 2.8 0:30 4. 0 3. 4

These components 'expresse'din the Seger formula are as follows:

0.500 Na 0 0.700 B203 5.233 sio 0.167 K20 0.700 A1 0 0.700 no, 0.333 CaO When intersolution was'complete thefrit was quenched in water. The frit was then milled, made into a slip, sprayed onto a vbisqued whiteware and dried according to the procedure described in Example 1. The dried bisqued whiteware was then fired to a temperature of 1075" C. according to the procedure described in Example 1 and was cooled to room temperature over a period of 14 hours. A smooth, faultless, glossy, white and opaque glaze was produced similar to those previously described.

Example 5 A frit was prepared from the following ingredients using the procedure described in Example 1:

Parts by weight Potters flint 163.5 Alumina 37.2 '[10 29.2 Borax 69.5 Soda ash 1.0 Red lead 39.6 Zinc carbonate 22.8

Calculated oxide composition of the frit was as follows:

Mole Weight percent percent S10 62. 8 51. 3 B 0 8. 4 8. 0 Also 8. 4 11. 6 TiOr- 8. 4 9. 1 N ago 4. 0 3. 4 P130. 4. 0 12. 2 ZnO 4. 0 4. 4 100. 0 100. 0

These components expressed in the Seger formula are as follows:

0333 N320 0.700 B203 5.233 sio 0.333 K20 0.700 A1203 0.700 r10 0.333 C210 7 When intersolution was complete the frit was quenched in water. The frit was then milled, made into a. slip, sprayed onto a decorative terra cotta body and dried according to the procedure described in Example 1. The dried decorative terra cotta body was then fired to a temperature of 925 C. according to the procedure described in Example 1 and was cooled to room temperature over a period of 14 hours. The resulting glaze was again similar to that described in Example 1.

Example 6 The following ingredients were fused to form a frit according to the procedure described in Example 1.

Parts by weight Potters flint 163.5

Alumina 37.2 T10, 29.2 Borax 69.5 Soda ash 1.0 Zinc car 45.5

4 The corresponding calculated oxide composition was:

Mole Weight percent percent S'O 62. 8 55. 6 E 10 s. 4 8.6 A1103.-- 8.4 12.6 TiO; 8.4 9.9 NagO 4. 3. 7 ZnO 8.0 9. 6

These components expressed in the Seger formula are as follows:

0.333 Na O 0.700 B203 5.233 sio 0.667 ZnO 0.700 A1203 0.700 T10 When intersolution was complete the frit was quenched in water. The frit was then milled, made into a slip, sprayed onto a terra cotta body and dried according to the procedure described in Example 1. The dried terra cotta body was then fired to a temperature of 925 C. according to procedure described in Example 1 and was cooled to room temperature over a period of 14 hours. Substantially the same results as in Example 1 were obtained.

' Example 7 A frit was prepared according to the procedure described in Example 1 by mixing the following ingredients:

Parts by weight These components expressed in the Seger formula are as follows:

0.500 H O 0.700 B 0 5233 SiO 0.500 Na O 0.700 A1 0 0.700 TiO When intersolution was complete the frit was quenched in water. The frit was then milled, made into a slip, sprayed onto a terra cotta body and dried according to the procedure described in Example 1. The dried terra cotta body was then fired to a temperature of 925 C. according to the procedure described in Example 1 and was cooled to room temperature over a period of 14 hours. Substantially identical results to those of Example 1 were obtained.

Example 8 A frit was prepared using the procedure described in Example 1 by mixing the followingingredients:

. r r Parts by weight Potter's flint.

160.5 Alumina 36.3 TiO 28.5 Borax 68.0 Soda as 4.8 Potassium carbon 12.3 Magnesite 19.7 Calcite 9.1 Barium carbonate 17.4 Corresponding calculated oxide composition was:

7 Mole Weight percent percent 510 00.0 53.5 BQOI 8.0 8.3 A! 8.0 12.1 TiO-i 8.0 0.5 N320. 5. 0 4-6 K20 2.0 2.8 0:10-. 2.0, 1.7 MgO 5.0 3.0 13210.. 2.0 4.5

These components expressed in the Seger formula are as follows:

0.3125 Na O 0.500 B 0 3.750 SiO 0.125 K 0 0.500 A1 0 0.500 TiO 0.3125 MgO 0.125 CaO 0.125 BaO When intersolution was complete the frit was quenched in water. The frit was then milled, made into a slip, sprayed onto a bisqued tile slag and dried according to the procedure described in Example 1. The dried bisqued tile slag was then fired to a temperature of 1075 C. according to the procedure described in Example 1 and was cooled to room temperature over a period of 14 hours. The resulting glaze was substantially identical in character with those described in the previous examples.

Example 9 A frit was prepared using the procedure described in Example 1 by mixing the following ingredients:

Parts by weight The components expressed in the Seger formula areas follows:

0.167 K 0 0.333 CdO -When'intersolution was'completethe "frit was quenched 5.233 SiO v 0.700 A1 0 0.700 TiO sprayed onto an earthenware pottery body and dried p a. in water. The frit was then ,milled, madelinto a slip,

according to the procedure described in Example 1. The dried earthenware pottery body was then fired to a tem- .perature of 975 C. according to the procedure described in Example 1 and was cooled to room temperature over a period of 14 hours The resultingglaze .was similar to those described in'the previous 'examplesr A frit was prepared using the procedure described in Example 1 by mixing the following ingredients:

These components expressed in the Seger formula are as follows:

1.000 Na O 6.978 sio 0.933 A1203 0.933 no 0.333 zro When intersolution was complete the frit was quenched in water. The frit was then milled, made into a slip, sprayed onto a white ware tile panel and dried according to the procedure described in Example 1. The dried white ware tile panel was then fired to a temperature of 1075C. according to the procedure described in Example 1 and was cooled to room temperature over a period of 14 hours. The glaze produced was similar to those described in the previous examples.

Example 11 A frit was prepared using the procedure described in Example 1 by employing the following mixture:

Parts by weight Potters fiint 120.0 Alumina 20.4 TiO 32.0 Anhydrous borax 26.8 K CO 23.0 NaNO 28.3' The mixture had the following composition:

Mole Weight percent percent $10. 60.0 53.3 B203 8.0 8.2 TlOL 12. 0 14. 2 Au0= 6.0 9.0 N320 9.0 8.3 K10 5.0 7.0 100.0 100.0

'10, These components expressed in the Seger formula are as follows: 7

0.571 B 0 4.286 SiO,

When intersolutionwas complete the frit'was quenched in water. The frit .wasithen'mued;madeime'ai slip, sprayed onto a ceramic tile and dried according to the procedure described in Example 1. The dried ceramic tile was then fired to a temperature of 975 C. according to the procedure described in Example 1 and was cooled to room temperature over a period of 14 hours. The glaze was similar in appearance and texture to that previously described in Example 1.

Example 12 A frit was prepared from the following mixture using the procedure described in Example 1:

Parts by weight Potters flint 112.0 Alumina 34.0 Ti0 3.2.0 Anhydrous borax 26.8 K CO i 23.0 NaNO The mixture had the following composition:

Mole .Weight percent percent s10. 50. 0 4s. 0 3203- 8. 0 8. 0 Tioz 12. 0 13. 8 A1201 10. 0 14. 7 N820 9. 0 8. 1 K20 5. 0 6. 8 100. 0 100. 0

The components follows:

0.643 Na O l 0.571 B 0 4.000'SiO 'f. 0.357 K 0 0.714 A1 0 '0.85 7 TiQ;

expressed in the Seger formula areas When intersolution wascomplete the hit was quenched in water. The frit was then milled, made into a slip, sprayed onto a ceramic tile and dried according to the procedure described in Example 1. The dried ceramic tile was then fired to a temperature of 1050 C. according to the procedure described in Example 1 and was cooled to room temperature over a period of 14 hours. The glaze was similar in appearance and texture to that'previously described in Example 1. i

Example 13 Another frit was made with the following ingredients using the same procedure described in Example 1 Parts 'by weight Potter's flint 124.1 Alumina V 27.2 TiO 29.3 Anhydrous borax 26.8. Na CO 14.1

Strontium carbonate These constituents were smelted according to Example) and the smelted composition was quenched in water. The frit had the following oxide composition:

12 of Example 1 and quenched in Water to form the frit. The frit had the following oxide composition:

Mole Weight Mole Weight percent percent 5 percent percent I 810 02. 54. 9 SiO- 59. 0 53. 7 B20; 8. 0 8. 2 BZOB- 8. 0 8- 4 2 8.0 12.0 A1001 9. 5 14. 7 2 11.0 1?.0 iOu N a O 8. 0 .3 a2 B a- 3.0 4.6 10 F 2.0 1.1

These constituents expressed in the Seger formula are as follows:

0.727 Na O 0.727 B203 5.636 sio 0.273 SrO 0.727 A1203 1.000 no The frit formed was then ball milled, screened and dried according to the procedure of Example 1 using the same mill additions. The slip formed was then sprayed onto a tile body as in Example 1, dried and fired at 1050" C. for /2 hour and cooled to room temperature over a 14 hour period. The glaze was similar in appearance and texture to that previously described in Example 1.

Example 14 Another frit was prepared using the following ingredients:

Parts by weight Potters flint 119.1 Alumina 27.2 Ti0 21.3 Anhydrous borax 26. 8 Soda ash 33 .6 Nm2PO4H2O i This mixture was smelted according to the procedure of Example 1 and quenched in water to form the frit. The frit had the following oxide composition:

These constituents expressed in the Seger formula are as follows:

1.000 Na O 0533 B203 3.967 510 0.533A1 o 0.533 "no 0.100 P205 The frit was ball milled, screened, dried and sprayed onto a tile body in the same manner as that described in Example 1 and fired at 975 C. for '1 hour to mature the glaze. After cooling to room temperature in 14 hours, the glaze had the same appearance and texture as the glaze described in Example 1.

Example 15 Another frit was prepared using the following ingredients:

Parts by weight .Potters flint 1 18.1 Alumina 32.3 Ti0 18.6 Anhydrous borax 26.8 Soda ash 30.0 NaF 5.6

This mixture .was-sm'elted according to the procedure These constituents expressed in the Seger formula are as follows:

1.000 N5 0 0.551 12,0 4.069 sio 0.655 A1203 0.483 no The frit was ground, screened, dried and sprayed onto a tile body in the same manner as that described in Example 1 and fired at 1000 C. for /2 hour to mature the glaze. The glaze produced had the same texture and appearance as the glaze produced in Example 1.

The ceramic glazes of the present invention may be applied to a wide variety of ceramic materials having coeflicients of thermal expansion in the range characteristic of American wall tile. They are of excellent white color and opacity, and are characterized by good acid and alkali resistance. The glazes herein described, moreover, mature at relatively low temperatures, and thus help in the avoidance of warping or deformation of the substrate body at high temperatures. Being of recrystallizing variety, they provide the most efiicient possible use of the titania opacifier and therefore give excellent opacification' even at relatively low application weights.

While this invention has been described and illustrated by the examples shown, it is not intended to be strictly limited thereto, and other variations and modifications may be employed within the scope of the following claims.

We claim:

1. An opacified glaze composition containing as the opacifying agent recrystallized titanium dioxide, said glaze composition prepared by maturing a ceramic frit composition on a ceramic substrate, said glaze composition consisting essentially of SiO in amount from 50 to 65 mole percent, B 0 in amount from 6 to 12 mole percent, TiO in amount from 5 to 25 mole percent, A1 0 in amount from 5 to 14 mole percent, Na O in amount from 4 to 9 mole percent and at least one additional fluxing oxide selected from the group consisting of K 0 in amount up to 8 mole percent, CaO in amount up to 4 mole percent, MgO in amount up to 8 mole percent, BaO in amount up to 6 mole percent, SrO in amount up to 4 mole percent, PhD in amount up to 6 mole percent, ZnO in amount up to 10 mole percent, ZrO in amount up to 5 mole percent, L in amount up to 8 mole percent, and CdO in amount up to 4 mole percent, the sum of said Na O and said additional fluxing oxide being from 10 to 18 mole percent.

2. A composition according to claim 1 in which the TiO is present in amount from 5 to 16 mole percent.

3. A composition according to claim 1 in which the glaze also contains from 1 to 2 mole percent of P 0 4. A composition according to claim 1 in which the glaze also contains from 1 to 5 mole percent F References Cited in the file of this patent UNITED STATES PATENTS 2,414,633 Bryant Jan. 21, 1947 2,483,393 Baldwin Oct. 4, 1949 2,640,784 -Tiede et a1. June 2, 1953 2,662,020 Schofield et a1 Dec. 8, 1953 2,753,271 Treptow July 3, 1956 

1. AN OPACIFIED GLAZE COMPOSITION CONTAINING AS THE OPACIFYING AGENT RECRYSTALLIZED TITANIUM DIOXIDE, SAID GLAZE COMPOSITION PREPARED BY MATURING A CERAMIC FRIT COMPOSITION ON A CERAMIC SUBSTRATE, SAID GLAZE COMPOSITION CONSISTING ESSENTIALLY OF SIO2 IN AMOUNT FROM 50 TO 65 MOLE PERCENT, B2O3 IN AMOUNT FROM 6 TO 12 MOLE PERCENT, TIO2 IN AMOUNT FROM 5 TO 25 MOLE PERCENT, AL2O3 IN AMOUNT FROM 5 TO 14 MOLE PERCENT, NA2O IN AMOUNT FROM 4 TO 9 MOLE PERCENT AND AT LEAST ONE ADDITIONAL FLUXING OXIDE SELECTED FROM THE GROUP CONSISTING OF K2O IN AMOUNT UP TO 8 MOLE PERCENT, CAO IN AMOUNT UP TO 4 MOLE PERCENT, MGO IN AMOUNT UP TO 8 MOLE PERCENT, BAO IN AMOUNT UP TO 6 MOLE PERCENT, SRO IN AMOUNT UP TO 4 MOLE PERCENT, PBO IN AMOUNT UP TO 6 MOLE PERCENT, ZNO IN AMOUNT UP TO 10 MOLE PERCENT, ZRO2 IN AMOUNT UP TO 5 MOLE PERCENT, LI2O IN AMOUNT UP TO 8 MOLE PERCENT, AND CDO IN AMOUNT UP TO 4 MOLE PERCENT, THE SUM OF SAID NA2O AND SAID ADDITIONAL FLUXING OXIDE BEING FROM 10 TO 18 MOLE PERCENT. 